1. Field of the Invention
The present invention relates to a dual tone multifrequency (hereinafter, referred to as DTMF) signal receiving apparatus equipped with a DTMF signal judging circuit for judging whether a signal inputted into a facsimile apparatus, a multi-function telephone or the like is a DTMF signal or not.
2. Description of the Related Art
A DTMF signal is used, for example, for transmitting a phone number or the like from a pushbutton telephone to a telephone central office. A DTMF signal includes one of four tones each having a frequency in a low frequency band and one of four tones each having a frequency in a high frequency band.
In general, the following frequencies are allocated for four tones in the low frequency band: fa=697 Hz; fb=770 Hz; fc=852 Hz; and fd=941 Hz.
For four tones in the high frequency band, the following frequencies are allocated: fe=1209 Hz; ff=1336 Hz; fg=1477 Hz; and fh=1633 Hz.
For example, a DTMF signal indicating the numeral "1" of a phone number includes a tone having a frequency of 697 Hz (low frequency) and a tone having a frequency of 1209 Hz (high frequency).
Since such a DTMF signal is also used for transmitting data after a line connection, some facsimile apparatuses or some multi-function telephones connected to a public telephone network are equipped with a DTMF signal receiver.
FIG. 4 is a block diagram of a conventional DTMF signal receiver. The DTMF signal receiver includes eight band-pass filters 110 to 117, which are all connected to an input terminal 100. The band-pass filters 110 to 117 correspond to the above-mentioned eight tones (four having a low frequency and four having a high frequency), respectively. The frequency of each of the eight tones corresponds to a center frequency of a passband of the corresponding band-pass filter.
The band-pass filters 110 to 117 extract frequency components of a signal inputted into the input terminal 100, the frequency components corresponding to the eight tones, respectively.
The band-pass filters 110 to 117 are connected to multipliers 120 to 127, respectively, for squaring outputs from the band-pass filters 110 to 117. The multipliers 120 to 127 are connected to smoothing low-pass filters 130 to 137 for smoothing outputs from the multipliers 120 to 127. The low-pass filters 130 to 137 are connected to level detectors 140 to 147 for detecting power levels of outputs from the smoothing low-pass filters 130 to 137. The level detectors 140 to 147 are all connected to a coder 150 for outputting a code based on outputs from the level detectors 140 to 147.
In such a conventional DTMF signal receiver, when a signal which is not a DTMF signal but includes at least one frequency component identical with one of the DTMF frequency components, for example, an audio signal or white noise is inputted, such a signal is erroneously recognized as a DTMF signal. Thus, the DTMF signal receiver malfunctions and a system using the DTMF signal receiver is abnormally operated.